Torstatus: What does stable server mean?

[Klaus Layer]

Hi,

since several days I am running a tor relay. The torstatus page at i.e. 
http://torstatus.blutmagie.de shows for some of the listed servers the 
attributes "stable server" and "guard". What does that mean? When does a tor 
relay becomes a stable server or a guard?

Thanks,

Klaus

-- 
Klaus Layer
GPG Fingerprint: 466D 12F8 28A3 D137 A77E FC3B 271C 2D79 6F5E 94C9
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